1. Field of the Invention
The present invention relates to a novel and useful powder coating composition.
More particularly, the present invention relates to a powder coating composition, which can ensure excellent smoothness, scratch resistance, water resistance and humidity resistance, even when formed into a thin film having a thickness smaller than that employed conventionally in various purposes.
2. Description of Related Art
Powder coating compositions have widely been used as an environmentally friendly coating composition, which does not have an organic solvent to evaporate into the air during coating, in the entirety of the metal coating field. Among these powder coating compositions, a powder coating composition comprising an epoxy group-containing vinyl copolymer as a resin component and a polycarboxylic acid compound as a curing agent component is widely used, because it can form a coating film having excellent weather resistance, and is disclosed, for example, in Japanese Patent Application, First Publication No. 52-69936. However, powder coating compositions including one described in the publication were generally unsatisfactory in finishing appearance of the coating film as compared with a solvent-based coating.
Powder coating compositions for use as a next generation automobile top clear coating composition have recently been researched, intensively, and the demand for the finishing appearance of the coating film has increased more than before and the scratch resistance has been required as a new required performance.
To meet these demands, a powder coating composition having excellent finishing appearance and scratch resistance, and a coating method therefor are disclosed (U.S. Pat. No. 5,948,866). When using such a powder coating composition, a coating film has excellent finishing appearance and scratch resistance. Therefore, the feature of the powder coating composition increases the probability that it can be used as an automobile top clear coating composition, although the powder coating composition has hardly been used heretofore because of its poor finishing appearance and scratch resistance.
The powder coating composition is an environmentally friendly coating composition which can substantially eliminate organic solvent discharge to the air during coating. If the powder coating composition is exclusively used as the automobile top clear coating composition having a huge market, it becomes possible to markedly reduce burden on the global environment. From such a point of view, the powder coating composition has already started to partially replace conventional solvent-based coating compositions.
However, since excellent finishing appearance and scratch resistance of the coating film are required in this purpose, a conventional powder coating composition must be formed into a thick film at present in order to ensure the same performances as those of the solvent-based coating composition. Therefore, when a conventional powder coating composition is formed into a thin film, satisfactory performances cannot be achieved because of poor finishing appearance. If required performances can be achieved by forming the conventional powder coating composition into a thin film, it becomes possible to drastically reduce the total cost accompanying the amount of the powder coating composition, energy consumption and coating. Therefore, the demand of the market for conversion into a thin film has drastically increased.
Furthermore, the demand for coating film performances other than the finishing appearance and scratch resistance has increased more and more, thus causing a problem in that a powder coating composition is unsatisfactory in long-term water resistance and humidity resistance of the coating film under severe conditions. Therefore, a powder coating composition, which meets various properties, has never been obtained at present.